Vapor grown carbon fiber, and production method and use thereof

ABSTRACT

A vapor grown carbon fiber, each fiber filament of the carbonfiber having a branching degree of at least 0.15 occurrences/μm and a bulk density of 0.025 g/cm 3  or less and a producing method of the carbon fiber by spraying a raw material solution containing a carbon source and a transition metallic compound into a reaction zone and subjecting the raw material solution to thermal decomposition, which is characterized in (1) spraying the raw material solution at a spray angle of 3° to 30° and (2) feeding a carrier gas through at least one site other than an inlet through which the raw material solution is sprayed, and a composite material comprising the carbon fiber.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This is an application based on the prescription of 35 U.S.C. Section111(a) with claiming the benefit of filing date of U.S. ProvisionalApplication Ser. No. 60/426,400 filed Nov. 15, 2002 under the provisionof 35 U.S.C. 111(b), pursuant to 35 U.S.C. Section 119(e)(1).

TECHNICAL FIELD

The present invention relates to a method for producing carbon fiberthrough a vapor phase process. More particularly, the present inventionrelates to a method for producing carbon fiber having a large number ofbranches by thermally decomposing an organic compound through a vaporphase process, to carbon fiber produced through the method, and to acomposite material containing the carbon fiber.

BACKGROUND ART

In general, carbon fiber is dispersed in a matrix such as resin, tothereby impart electrical conductivity and thermal conductivity thereto.Vapor grown carbon fiber (hereinafter may be abbreviated as “VGCF”) isvery useful, since, even when a small amount of the carbon fiber isadded to a resin, the resultant resin composition exhibits greatlyenhanced electrical conductivity and thermal conductivity, and thereforeworkability of the resin composition is not lowered, and the surfaceappearance of a molded product formed from the composition is notimpaired (U.S. Pat. No. 5,643,990). As has been known, when carbon fiberhaving a large number of branches is added to a material, the electricalconductivity of the material is enhanced (WO 02/049412). Therefore,demand has arisen for production of carbon fiber having a large numberof branches.

As one method for producing carbon fiber through a vapor phase process,a gasification method has been proposed (U.S. Pat. No. 4,572,813). Inthe gasification method, a solution of an organic substance in which anorgano-transition metallic compound is dissolved is gasified, to therebyallow reaction to proceed at high temperature within a heating zone.This gasification method produces carbon fiber having a small number ofbranches. Meanwhile, there has been proposed a method for producingbranched carbon fiber by spraying droplets of a raw material onto thewall of a reaction tube (Japanese Patent No. 2778434). In this method,droplets of a raw material are fed to the reaction tube wall, to therebygrow carbon fiber on the reaction tube wall. After the reaction tubewall is covered with the thus-grown carbon fiber, droplets of the rawmaterial are sprayed onto the carbon fiber, a catalyst is generated onthe carbon fiber, and, on the carbon fiber serving as a substrate, freshcarbon fiber is grown to thereby form branches, whereby branched carbonfiber is produced at high yield.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide carbon fiber having aconsiderably large number of branches as compared with conventionalvapor grown carbon fiber through a producing method wherein the numberof catalyst particles that are effectively utilized for carbon fibergrowth is increased.

The present inventors have performed extensive studies on, for example,the method of feeding a raw material solution to a reaction zone of avapor grown carbon fiber production apparatus (reaction tube), and as aresult have found that carbon fiber having a large number of branchesand a low bulk density is obtainable when the raw material isefficiently fed to a reaction zone maintained at a high temperature. Thepresent invention has been accomplished on the basis of this finding.

Accordingly, the present invention provides a vapor grown carbon fiber,a method for producing the carbon fiber, and a composite materialcontaining the carbon fiber, which are described below.

-   1. A vapor grown carbon fiber, each fiber filament of the carbon    fiber having a branching degree of at least 0.15 occurrences/μm.-   2. A vapor grown carbon fiber characterized by comprising carbon    fiber filaments, each having a branching degree of at least 0.15    occurrences/μm, in an amount of at least 10 mass %.-   3. A vapor grown carbon fiber having a bulk density of 0.025 g/cm³    or less.-   4. The vapor grown carbon fiber according to 1 or 2 above, which has    a bulk density of 0.025 g/cm³ or less.-   5. The vapor grown carbon fiber according to any of 1 to 3 above,    which, when compressed so as to have a bulk density of 0.8 g/cm³,    has a specific resistance of 0.025 Ωcm or less.-   6. The vapor grown carbon fiber according to any of 1 to 3 above,    each fiber filament of the carbon fiber having a diameter of 1 to    500 nm.-   7. The vapor grown carbon fiber according to any of 1 to 3 above,    which is produced by feeding a raw material solution containing a    carbon source and a transition metallic compound into a reaction    zone through spraying at a spray angle of 3° to 30° and subjecting    the raw material solution to thermal decomposition.-   8. The vapor grown carbon fiber according to any of 1 to 6 above,    which is produced by feeding a raw material solution containing a    carbon source and a transition metallic compound into a reaction    zone through spraying, while feeding a carrier gas through at least    one site other than an inlet through which the raw material solution    is sprayed, and subjecting the raw material solution to thermal    decomposition.-   9. A method for producing a vapor grown carbon fiber comprising    spraying a raw material solution containing a carbon source and a    transition metallic compound into a reaction zone and subjecting the    raw material solution to thermal decomposition, characterized in    that the raw material solution is sprayed at a spray angle of 3° to    30°.-   10. The method for producing a vapor grown carbon fiber according to    9 above, wherein droplets of the raw material solution have an    average diameter of at least 5 I═.-   11. The method for producing a vapor grown carbon fiber according to    9 or 10 above, wherein the raw material solution and a carrier gas    are fed through a concentric multi-tube nozzle into a reaction tube.-   12. The method for producing a vapor grown carbon fiber according to    11 above, wherein the raw material solution is fed through one of    the tubes of the multi-tube nozzle, and another tube serves as a    passage for the carrier gas only.-   13. The method for producing a vapor grown carbon fiber according to    12 above, wherein the raw material solution and the carrier gas are    fed through the inner tube of concentrically disposed two tubes, and    the carrier gas is fed through the outer tube of the tubes.-   14. The method for producing a vapor grown carbon fiber according to    12 above, wherein the carrier gas is fed through the innermost tube    and the outermost tube of concentrically disposed three tubes, and    the middle tube of the tubes serves as a passage for the raw    material solution only.-   15. A method for producing a vapor grown carbon fiber comprising    spraying a raw material solution containing a carbon source and a    transition metallic compound into a reaction zone and subjecting the    raw material solution to thermal decomposition, characterized in    that a carrier gas is fed through at least one site other than an    inlet through which the raw material solution is sprayed.-   16. The method for producing a vapor grown carbon fiber according to    15 above, wherein the raw material solution is sprayed at a spray    angle of 3° to 30°.-   17. The method for producing a vapor grown carbon fiber according to    9 or 15 above, wherein the raw material solution containing a carbon    source and a transition metallic compound further contains a    surfactant and/or thickening agent.-   18. The method for producing a vapor grown carbon fiber according to    9 or 15 above, which comprises heating and firing recovered carbon    fiber in a non-oxidative atmosphere at 800° C. to 1,500° C. and    subsequently heating the thus-fired carbon fiber in a non-oxidative    atmosphere at 2,000 to 3,000° C., to thereby graphitize the carbon    fiber.-   19. The method for producing a vapor grown carbon fiber according to    18 above, wherein, before being graphitized through heating, the    recovered carbon fiber is doped with at least one boron compound,    serving as a crystallization facilitating compound, selected from    the group consisting of boron, boron oxide, boron carbide, a boric    ester, boric acid or a salt thereof, and an organic boron compound    in an amount of 0.1 to 5 mass % as reduced to boron.-   20. A composite material comprising a vapor grown carbon fiber    according to any of 1 to 8 above.-   21. A composite material comprising a vapor grown carbon fiber    produced through a method according to any of 9 to 19 above.-   22. A resin composition comprising a vapor grown carbon fiber    according to any of 1 to 8 above.-   23. A resin composition comprising a vapor grown carbon fiber    produced through a method according to any of 9 to 19 above.

Main raw materials (essential raw materials) employed for producing thecarbon fiber of the present invention are an organic compound and atransition metallic compound.

No particular limitations are imposed on the organic compound which maybe employed as a raw material of the carbon fiber, so long as theorganic compound assumes to be in a liquid form. Specific examples ofthe organic compound which may be employed include aromatic compoundssuch as benzene, toluene and xylene; linear-chain hydrocarbons such ashexane and heptane; cyclic hydrocarbons such as cyclohexane; alcoholssuch as methanol and ethanol; gasoline; and kerosene. Aromatic compoundsare preferred, with benzene being most preferred. These-carbon sourcesmay be employed singly or in combination of two or more species. In thecase of feeding of an organic compound, the entirety of the compound maybe fed in the form of droplets. Alternatively, a portion of the organiccompound may be fed in the form of droplets, and the remaining portionof the compound may be fed in the form of liquid or gas.

The transition metallic compound serving as a catalyst is preferably anorganic or inorganic compound containing a metal belonging to Group IVa,Va, VIIa, VIIa or VIII. Particularly, Fe compounds, Ni compounds and Cocompounds (e.g., ferrocene and nickelocene), which are transitionmetallic compounds that generate transition metal ultrafine seeds, arepreferred.

Productivity of the carbon fiber can be enhanced by adding a sulfursource serving as a promoter to the raw material solution. The sulfursource may be elemental sulfur, an organic sulfur compound such asthiophene or an inorganic sulfur compound such as hydrogen sulfide.However, from the viewpoint of handling, elemental sulfur and thiophene,which are dissolved in a carbon source, are preferred. These sulfursources (elemental sulfur and sulfur compounds) may be employed singlyor in combination of two or more species.

The raw material solution is prepared by dissolving a transitionmetallic compound in an organic compound. Droplets of the raw materialsolution are preferably generated through the spraying method shown inFIG. 3, which employs a spray nozzle.

Preferably, the raw material droplets are heated as quickly as possibleto the temperature of a reaction zone of a reactor, the temperaturebeing determined to be equal to or higher than the decompositiontemperature of the organic compound. This is because the decompositiontemperature of the transition metallic compound is generally lower thanthat of the organic compound, and therefore, when the raw materialdroplets are heated slowly, the transition metallic compound decomposesto thereby generate fine particles of the metal before growth of carbonfiber, and the resultant fine particles collide with one another and aregrown into large particles until they no longer exhibit a catalystfunction. Quick feeding of the raw material droplets to thehigh-temperature zone by use of a spray nozzle is effective forgenerating large amounts of catalyst particles which can be employed forgrowth of carbon fiber. During the course of feeding of the droplets, acritical point is to regulate the spray angle of the raw materialsolution or the diameter of each of the droplets by varying, forexample, the shape of the spray nozzle and the viscosity, surfacetension and density of the raw material solution.

Specifically, the spray angle of the raw material solution is preferably3° to 30°, more preferably 5° to 25°. As used herein, the term “sprayangle” refers to, as shown in FIG. 1, an angle θ (vertical angle) formedby the outermost trajectories of the raw material droplets with the tipportion of a nozzle serving as the vertex. When the spray angle exceeds30°, the droplets tend to collide against a reaction wall section whosetemperature is low, the temperature increasing rate of the dropletswhich do not reach a high-temperature section becomes low, and theamount of effective catalyst particles decreases, whereby the resultantcarbon fiber comes to have a small number of branches. In contrast, whenthe spray angle is less than 3°, the amount of the droplets which passthrough the high-temperature section increases, and the conversion rateof the raw material is lowered, leading to low yield of the carbonfiber.

Each of the raw material droplets preferably has a diameter of 5 μm ormore, more preferably 5 to 300 μm, much more preferably 10 to 100 μm.When the raw material droplet has a diameter of less than 5 μm, thegasification rate of the droplets increases and the droplets do notreach the high-temperature section. As a result, the temperatureincreasing rate of the droplets is lowered and the number of effectivecatalyst particles decreases, whereby the resultant carbon fiber comesto have a small number of branches. In contrast, when the raw materialdroplet has a diameter exceeding 300 μm, heating the raw materialrequires a long period of time, and thus the conversion rate of the rawmaterial is lowered. As used herein, the droplet diameter is measured bymeans of the Doppler method as follows. Specifically, the raw materialsolution is sprayed outside a reaction tube by causing air to flowthrough a spray nozzle; the thus-sprayed droplet particles areirradiated with two crossed laser beams; light scattered by theparticles that have passed through interference fringes is detected by alight-receiving device provided at a certain location; and the diametersof the particles are calculated on the basis of the phase difference.The average of the thus-calculated diameters of the particles is takenas the droplet diameter.

No particular limitations are imposed on the shape of the nozzle, solong as the droplet diameter and the spray angle fall withinpredetermined ranges. Preferably, the nozzle has a structure such thatthe droplet diameter and the spray angle can be readily regulated.

Specifically, there may be employed a nozzle having, for example, aconcentric multi-tube structure, a single-fluid-type structure or adouble-fluid-type structure (an interior mixing type in which a reactantsolution and a carrier gas are mixed in the interior of a nozzle, or anexterior mixing type in which a reactant solution and a carrier gas aremixed outside a nozzle). Particularly, a nozzle having a concentricmulti-tube structure or a double-fluid-type structure is preferred. Whena double-fluid-type nozzle is employed, the droplet diameter can beregulated by varying the feed amount of the raw material solution or acarrier gas, and the spray angle can be regulated by varying thestructure of the nozzle.

Specific examples of the nozzle of concentric multi-tube structure whichmay be employed include a nozzle of double-tube structure (its verticalcross-sectional view is shown in FIG. 2(A)) and a nozzle of triple-tubestructure (its vertical cross-sectional view is shown in FIG. 2(B)). Inorder to regulate the spray angle of the raw material solution,preferably, at least a portion of a carrier gas (3) to be fed to areaction tube (1) is fed through a tube other than a tube through whichthe raw material solution (4) is fed. In the case where a nozzle ofdouble-tube structure is employed, when the raw material solution (4)and a carrier gas (hydrogen) (3) are fed through the inner tube (5), aportion of the carrier gas (3) is fed through the outer tube (6), andthe spray angle can be readily regulated by increasing the amount ofhydrogen (3) fed through the outer tube (6). At the spraying side (8) ina nozzle of double-tube structure, the inner tube may be longer orshorter than the outer tube. Employing the inner tube longer than theouter tube is preferable, as the spray angle is readily regulated. Inthe case of a nozzle of double-tube structure (2), the diameter of theinner tube is preferably 0.01 to 2 mm, more preferably 0.1 to 0.5 mm,and the clearance between the outer tube and the inner tube (d) ispreferably 0.01 to 2 mm, more preferably 0.1 to 0.5 mm. When thediameter of the inner tube and the clearance between the outer tube andthe inner tube exceed 2 mm, the raw material solution fails to besprayed normally, and carbon fiber may fail to be generated as a resultof growth of catalyst particles, whereas when the diameter of the innertube and the clearance between the outer tube and the inner tube areless than 0.01 mm, the feed amounts of the raw material and the carriergas fail to be increased, and thus productivity of carbon fiber islowered.

In the case where a nozzle of triple-tube structure is employed, acarrier gas (3) is fed through the innermost tube (5) and the outermosttube (6), and the raw material solution is fed through the middle tube(7). In this case, when the rate of the carrier gas fed through theinnermost and outermost tubes is regulated, the spray angle of the rawmaterial solution can be readily regulated so as to fall within a rangeof 3° to 30°. At the spraying side (8) in a nozzle of triple-tubestructure, the length of the innermost, middle and outermost tubes maybe different. Employing the middle tube longer than the outermost tubeis preferable, as the spray angle is readily regulated. As in the caseof a nozzle of double-tube structure, the diameter of the innermost tube(5), the clearance between the outermost tube (6) and the middle tube(7), and the clearance between the middle tube and the innermost tubeare preferably 0.01 to 2 mm, more preferably 0.1 to 0.5 mm.

Generally, the droplet diameter varies depending on the viscosity,surface tension and density of the solution to be sprayed. The dropletdiameter can be regulated to a desirable size by adding a thickeningagent, surfactant, etc. to the raw material solution.

Generally, the droplet diameter becomes larger when the viscosity of theraw material solution increases. Therefore, adding a thickening agent tothe raw material solution enables to feed the raw material droplets tothe high-temperature zone. There is no particular limitation on athickening agent as long as it has a higher viscosity than that of theorganic compound of the raw material and can be dissolved in theraw-material organic compound. Specifically, mineral oil, vegetable oil,vegetable fat, paraffin, fatty acids (oleic acid, linolic acid, etc.),fatty alcohol (decanol, octanol, etc.), polymer (polyvinylalcohol,polyethyleneglycol, polypropyleneglycol, etc.) are used.

As a surfactant, cation surfactant, anion surfactant, nonionicsurfactant and ampholytic surfactant can be used. Desirable surfactantsinclude CnH_(2n+1)SO₃M (n=8 to 16, M=Na, K, Li, N(CH₃)₄),C_(n)H_(2n+1)SO₄M (n=8 to 16, M=Na, K, Li, N(CH₃)₄)(C_(n)H_(2n+1))₂COOCH₂COOCHSO₃M (n=8 to 16, M=Na, K, Li, N(CH₃)₄)C_(n)H_(2n+1)N(CH₃)₃X (n=8 to 15, X=Br, Cl, I),C_(n)H_(2n+1)N(CH₃)₂CH₂COO (n=8 to 15), CnH_(2n+1)CHOHCH₂OH (n=8 to 15)and C_(n)H_(2n+1)(OC₂H₄)_(m)HCH₂OH (n=8 to 15, m=3 to 8).

In order to feed the raw material and a transition metallic compoundserving as a catalyst to a thermal decomposition zone for developing andmaintaining the activity of the catalyst, a carrier gas containing atleast a reducing gas such as hydrogen gas is employed. The amount of thecarrier gas is appropriately 1 to 100 parts by mol on the basis of 1.0part by mol of an organic compound serving as a carbon source.

No particular limitations are imposed on the location at which thecarrier gas is brought into the reaction tube. As shown in FIG. 3, whenhydrogen gas is fed through at least one inlet (preferably four inlets)other than the inlet through which the raw material solution is fed, thegas in the reaction tube develops turbulence and transfer of heat fromthe reaction tube wall is promoted, leading to an increase in yield ofcarbon fiber.

A vertical electric furnace is generally employed as a reaction furnace.The temperature of the reaction furnace is 800 to 1,300° C., preferably1,000 to 1,300° C. The raw material solution and a carrier gas are fedto the reaction furnace heated to a predetermined temperature so as toallow reaction to proceed, thereby producing carbon fiber.

The thus-produced carbon fiber is preferably subjected to heat treatmentfor removal of volatile components and for graphitization of the carbonfiber. Removal of volatile components is carried out by recovering thecarbon fiber containing branched carbon fiber filaments produced in thereaction furnace and then heating and firing the carbon fiber at 800° C.to 1,500° C. in a non-oxidative atmosphere such as argon gas.Subsequently, the thus-treated carbon fiber is further heated at 2,000to 3,000° C. in a non-oxidative atmosphere to thereby allowgraphitization to proceed. During the course of graphitization, thecarbon fiber is doped with a small amount of a crystallizationfacilitating element to thereby enhance crystallinity of the fiber. Thecrystallization facilitating element is preferably boron. Since thesurface of the thus-graphitized fine carbon fiber is covered with adense basal plane (a plane of hexagonal network structure), preferably,carbon fiber of low crystallinity which has been heated at 1,500° C. orlower is doped with boron. Even when carbon fiber of low crystallinityis employed, carbon fiber of high crystallinity can be obtained sincethe carbon fiber is heated to its graphitization temperature when beingdoped with boron; i.e., when being subjected to boronization.

The doping amount of boron is generally 5 mass % or less on the basis ofthe entire amount of carbon. When carbon fiber is doped with boron in anamount of 0.1 to 5 mass %, the crystallinity of the carbon fiber can beeffectively enhanced. Therefore, elemental boron or a boron compound(e.g., boron oxide (B₂O₃), boron carbide (B₄C), a boric ester, boricacid (H₃BO₃) or a salt thereof or an organic boron compound), whichserves as a crystallization facilitating compound, is added to carbonfiber such that the boron content of the carbon fiber falls within theabove range. In consideration of the conversion rate, the amount of theboron compound as reduced to boron is 0.1 to 5 mass % on the basis ofthe entire amount of carbon. It should be noted that the key requirementis that boron be present when the fiber is crystallized through heattreatment. Boron may be evaporated during the course of, for example,high-temperature treatment performed after carbon fiber has been highlycrystallized, and thus the boron content of the carbon fiber may becomelower than the amount of boron initially added to the fiber. Such a dropis acceptable so long as the amount of boron (B) remaining in thethus-treated carbon fiber is about 0.01 mass % or more.

The temperature required for introducing boron into carbon crystals orthe surface of carbon fiber is 2,000° C. or higher, preferably 2,300° C.or higher. When the heating temperature is lower than 2,000° C.,introduction of boron becomes difficult because of low reactivitybetween boron and carbon. Heat treatment is carried out in anon-oxidative atmosphere, preferably in an atmosphere of a rare gas suchas argon. When heat treatment is carried out for an excessively longperiod of time, sintering of carbon fiber proceeds, resulting in lowyield. Therefore, after the temperature of the center portion of carbonfiber reaches the target temperature, the carbon fiber is maintained atthe target temperature within one hour.

The carbon fiber produced through the method of the present inventionhas a large number of branches and thus readily forms a strong fibernetwork. Therefore, even when a small amount of the carbon fiber isadded to a matrix such as resin, the electrical conductivity and thermalconductivity of the matrix are enhanced. When the carbon fiber producedthrough the method of the present invention is compressed into acompact, the carbon fiber compact exhibits low specific resistance,since a strong fiber network is formed. The carbon fiber producedthrough the method of the present invention has a low bulk density, andfilaments of the fiber are not strongly entangled with one another.Therefore, the carbon fiber is characterized in exhibiting a gooddispersity when mixed in a material such as resin.

In the present invention, the diameter and branching degree of eachfiber filament of the carbon fiber are obtained through observation ofthe filament under an electron microscope. The branching degree (b/ΣL)is calculated from the sum of the lengths (ΣL) of the carbon fiberfilaments and the total branching points (b) of the filaments, bothbeing measured within a field of view. That is, the branching degree isdefined by the number of branching points per unit filament length. Acharacteristic feature of the carbon fiber of the present inventionresides in that each fiber filament of the fiber has a branching degreeof 0.15 occurrences/m or more. Preferably, a branching degree is between0.15 occurrences/μm and 10 occurrences/μm and more preferably between0.15 occurrences/μm and 1 occurrences/μm. In the case where thebranching degree is less than 0.15 occurrences/μm, when a small amount(about 1 mass %) of the carbon fiber is added to a material, theelectrical conductivity of the material is barely enhanced. From theviewpoint of enhancement of electrical conductivity, preferably, thecarbon fiber contains carbon fiber filaments having such a branchingdegree in an amount of 10 mass % or more.

The conventional gasification method produces carbon fiber containingsubstantially no branching portions, and conventional vapor grown carbonfiber (VGCF) has a branching degree of less than 0.15 occurrences/μm.When a small amount of such carbon fiber is added to a material, theelectrical conductivity of the material is barely enhanced.

Another characteristic feature of the carbon fiber of the presentinvention resides in that the carbon fiber has a bulk density of 0.025g/cm³ or less. Preferably, the bulk density is between 0.01 g/cm³ and0.025 g/cm³. In order to enhance reproducibility of measurement, thebulk density of the carbon fiber is obtained through the followingprocedure: the produced carbon fiber is heated in an argon atmosphere at1,000° C. for 15 minutes and then vibrated by use of a vibrationapparatus for one minute to thereby prepare a measurement sample; thesample (1 g) is placed into a 100-ml messcylinder and a microspatula isinserted thereinto, and the sample is stirred through vibration by useof a test tube Touch mixer for one minute; the resultant sample isstirred manually 10 times, and the microspatula is removed from themesscylinder, followed by vibration of the messcylinder by use of theTouch mixer for one minute; and the volume of the sample is measured andthe bulk density is calculated from the mass and the volume of thesample.

Carbon fiber produced through the conventional gasification method has abulk density of about 0.03 g/cm³, and conventional vapor grown carbonfiber (VGCF) has a bulk density of about 0.04 g/cm³. When a small amountof such carbon fiber is added to a material, the electrical conductivityof the material is barely enhanced.

Since the carbon fiber of the present invention is in a fibrous form,the carbon fiber is compressed into a compact having a bulk density of0.8 g/cm³, and the compact is subjected to measurement of specificresistance to determine the specific resistance of the carbon fiber. Thespecific resistance of the carbon fiber is preferably 0.025 Ωcm or less.In the case where the specific resistance is above 0.025 Ωcm, theelectrical conductivity of the material is barely enhanced when a smallamount (about 1 mass %) of the carbon fiber is added to a material.

No particular limitations are imposed on the diameter of each fiberfilament of the carbon fiber, but the diameter is preferably 1-500 nm orthereabouts, more preferably 5 to 200 nm, from the viewpoint ofenhancement of electrical conductivity.

The carbon fiber produced through the method of the present inventionhas a high branching degree, and thus exhibits excellent characteristicssuch as high electrical conductivity and high thermal conductivity.Therefore, when the carbon fiber is mixed with a matrix such as resin,metal or ceramic to thereby prepare a composite material, the matrixexhibits, for example, enhanced electrical conductivity and thermalconductivity.

Examples of the resin which may be employed in the composite materialinclude thermoplastic resins and thermosetting resins. Specific examplesinclude polyethylene (PE), polypropylene, nylon, urethane, polyacetal,polyphenylene sulfide, polystyrene, polycarbonate, polyphenylene ether,polyethylene terephthalate, polybutylene terephthalate, polyarylate,polysulfone, polyethersulfone, polyimide, polyoxybenzoyl, polyetherether ketone, polyetherimide, Teflon (registered trademark), siliconresin, cellulose acetate resin, ABS resin, AES resin, ABMS resin, AASresin, phenol resin, urea resin, melamine resin, xylene resin, diallylphthalate resin, epoxy resin, aniline resin and furan resin.

Examples of the ceramic matrix include aluminum oxide, mullite, siliconoxide, zirconium oxide, silicon carbide and silicon nitride.

Examples of the metal matrix include gold, silver, aluminum, iron,magnesium, lead, copper, tungsten, titanium, niobium, hafnium, alloysthereof and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of the spray angle of a raw materialsolution.

FIGS. 2(A) and 2(B) are vertical cross-sectional views showing thestructures of a double-tube raw material feed nozzle and a triple-tuberaw material feed nozzle, respectively, employed in the method of thepresent invention.

FIG. 3 shows an example of bringing a hydrogen carrier gas into areaction tube through a site other than an inlet through which a rawmaterial solution is sprayed.

FIG. 4 is a schematic cross-sectional view showing a vapor grown carbonfiber production system employed in Comparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described with reference to Examplesand Comparative Examples, but the present invention is not limited tothe below-described Examples.

EXAMPLE 1

Ferrocene (0.83 kg) and sulfur (0.059 kg) were dissolved in benzene (14kg) to thereby prepare a raw material solution (ferrocene content of thesolution: 5.5 mass %, sulfur content of the solution: 0.39 mass %).Nitrogen gas was caused to flow through a reaction furnace system (1)shown in FIG. 3, which includes a vertical heating furnace (1) (innerdiameter: 370 mm, length: 2,000 mm) whose top portion is equipped with araw material feed nozzle (spray nozzle) (2) (SU11, product of SprayingSystems Co.), to thereby purge oxygen gas from the furnace system.Subsequently, hydrogen gas was caused to flow through the furnace systemto thereby fill the furnace system with hydrogen gas. Thereafter, thetemperature of the reaction furnace was raised to 1,250° C. By use of apump, the raw material solution (130 g/min) and hydrogen gas (20 L/min)were fed through the raw material feed nozzle, and hydrogen gas (400L/min) was fed through a flange (9) provided on the upper portion of thereaction furnace. The spray angle of the raw material solution and theaverage diameter of the sprayed droplets were 21° and 30 μm,respectively. Under the above conditions, reaction was allowed toproceed for one hour to thereby produce carbon fiber. The resultantcarbon fiber was found to have a bulk density of 0.021 g/cm³. The carbonfiber was found to have a specific resistance of 0.0236 Ωcm when beingcompressed so as to have a bulk density of 0.8 g/cm³.

The thus-produced carbon fiber was observed under an electronmicroscope, and the average diameter of fiber filaments of the carbonfiber was found to be about 80 nm. The branching degree of each fiberfilament was measured, and found to be 0.3 occurrences/m. The mass ofthe carbon fiber was measured and carbonization yield (the mass of theproduced carbon fiber/the mass of the fed benzene) was calculated to be55%.

EXAMPLE 2

The vapor grown carbon fiber produced in Example 1 was fired at 1,000°C. for 15 minutes and then graphitized at 2,800° C. for 15 minutes. Thethus-graphitized carbon fiber was dispersed in polyacetal by use of akneader to thereby prepare a compound. The vapor grown carbon fiber wasadded to the resin in an amount of 5 mass %. The resultant compound wasmolded into a product by use of a thermal press and the volumeresistivity of the molded product was measured by means of thefour-terminal method. The volume resistivity was found to be 300 Ωm.

EXAMPLE 3

Nitrogen gas was caused to flow through a reaction furnace system (1)shown in FIG. 3, which includes a vertical heating furnace (innerdiameter: 370 mm, length: 2,000 mm) whose top portion is equipped with adouble-tube raw material feed nozzle having a structure shown in FIG.2(A), to thereby purge oxygen gas from the furnace system. Subsequently,hydrogen gas was caused to flow through the furnace system to therebyfill the furnace system with hydrogen gas. Thereafter, the temperatureof the reaction furnace was raised to 1,250° C.

By use of a pump, a raw material solution (a benzene solution containingferrocene in an amount of 4.5 mass % and sulfur in an amount of 0.32mass %) (50 g/min) and hydrogen gas (5 L/min) were fed through the innertube (5) of the raw material feed nozzle, hydrogen gas (10 L/min) wasfed through the outer tube (6) of the nozzle, and hydrogen gas (200L/min) was fed through a flange (9) provided on the upper portion of thereaction furnace. The spray angle of the raw material solution and theaverage diameter of the sprayed droplets were 26° and 30 μm,respectively. Under the above conditions, reaction was allowed toproceed for one hour to thereby produce carbon fiber. The resultantcarbon fiber was found to have a bulk density of 0.022 g/cm³. The carbonfiber was found to have a specific resistance of 0.027 Ωcm when beingcompressed so as to have a bulk density of 0.8 g/cm³. The thus-producedcarbon fiber was observed under an electron microscope, and the averagediameter of fiber filaments of the carbon fiber was found to be about100 nm. The branching degree of each fiber filament was measured andfound to be 0.3 occurrences/μm. The mass of the carbon fiber wasmeasured, and carbonization yield (the mass of the carbon fiber/the massof the fed benzene) was calculated to be 60%.

EXAMPLE 4

Nitrogen gas was caused to flow through a reaction furnace system (1)shown in FIG. 3, which includes a vertical heating furnace (innerdiameter: 130 mm, length: 2,000 mm) whose top portion is equipped with adouble-tube raw material feed nozzle having a structure shown in FIG.2(A), to thereby purge oxygen gas from the furnace system. Subsequently,hydrogen gas was caused to flow through the furnace system to therebyfill the furnace system with hydrogen gas. Thereafter, the temperatureof the reaction furnace was raised to 1,250° C.

By use of a pump, a raw material solution (a benzene solution containingferrocene in an amount of 7 mass % and sulfur in an amount of 0.5 mass%) (18 g/min) and hydrogen gas (5 L/min) were fed through the inner tube(5) of the raw material feed nozzle, hydrogen gas (10 L/min) was fedthrough the outer tube (6) of the nozzle, and hydrogen gas (450 L/min)was fed through a flange (9) provided on the upper portion of thereaction furnace. The spray angle of the raw material solution and theaverage diameter of the sprayed droplets were 26° and 20 μm,respectively. Under the above conditions, reaction was allowed toproceed for one hour to thereby produce carbon fiber. The resultantcarbon fiber was found to have a bulk density of 0.049 g/cm³. The carbonfiber was found to have a specific resistance of 0.042 Ωcm when beingcompressed so as to have a bulk density of 0.8 g/cm³.

The thus-produced carbon fiber was observed under an electronmicroscope, and the average diameter of fiber filaments of the carbonfiber was found to be about 9 nm. The branching degree of each fiberfilament was measured and found to be 0.2 occurrences/μm. The mass ofthe carbon fiber was measured and carbonization yield (the mass of theproduced carbon fiber/the mass of the fed benzene) was calculated to be15%.

EXAMPLE 5

Ferrocene (1 kg), sulfur (0.05 kg) and polypropyleneglycol (D-400,product of NOF Corporation, molecular weight: 400, decompositiontemperature: 290° C.) (0.5 kg) were dissolved in benzene (13.5 kg) tothereby prepare a raw material solution (containing ferrocene, sulfurand polyporpyleneglycol in the amount of 7 mass %, 0.4 mass % and 3 mass% respectively). Nitrogen gas was caused to flow through a reactionfurnace system (1) shown in FIG. 3, which includes a vertical heatingfurnace (1) (inner diameter: 370 mm, length: 2,000 mm) whose top portionis equipped with a raw material feed nozzle (spray nozzle) (2) (SU11,product of Spraying Systems Co.), to thereby purge oxygen gas from thefurnace system. Subsequently, hydrogen gas was caused to flow throughthe furnace system to thereby fill the furnace system with hydrogen gas.Thereafter, the temperature of the reaction furnace was raised to 1,250°C.

By use of a pump, a raw material solution (230 g/min) and hydrogen gas(5 L/min) and hydrogen gas (20 L/min) were fed through the raw materialfeed nozzle, and hydrogen gas (400 L/min) was fed through a flange (9)provided on the upper portion of the reaction furnace. The spray angleof the raw material solution was 21° and the average diameter of thesprayed droplets was 40 am. Under the above conditions, reaction wasallowed to proceed for one hour to thereby produce carbon fiber. Theresultant carbon fiber was found to have a bulk density of 0.024 g/cm³.The carbon fiber was found to have a specific resistance of 0.024 Ωcmwhen being compressed so as to have a bulk density of 0.8 g/cm³.

The thus-produced carbon fiber was observed under an electronmicroscope, and the average diameter of fiber filaments of the carbonfiber was found to be about 80 nm. The branching degree of each fiberfilament was measured and found to be 0.4 occurrences/μm. The mass ofthe carbon fiber was measured and carbonization yield (the mass of thecarbon fiber/the mass of the fed benzene) was calculated to be 57%.

COMPARATIVE EXAMPLE 1

Carbon fiber was produced by use of a system shown in FIG. 4, whichincludes a vertical heating furnace (inner diameter: 370 mm, length:2,000 mm) whose top portion is equipped with a double-fluid-typehollowcone raw material feed nozzle. Nitrogen gas was caused to flowthrough the furnace system to thereby purge oxygen gas from the furnacesystem. Subsequently, hydrogen gas was caused to flow through thefurnace system to thereby fill the furnace system with hydrogen gas.Thereafter, the temperature of the reaction furnace was raised to 1,250°C.

By use of a pump, a raw material solution (a benzene solution containingferrocene in an amount of 5.5 mass % and sulfur in an amount of 0.39mass %) (130 g/min) and hydrogen gas (20 L/min) were fed through the rawmaterial feed nozzle. The spray angle of the raw material solution was60°. Under the above conditions, reaction was allowed to proceed for onehour to thereby produce carbon fiber. The resultant carbon fiber wasfound to have a bulk density of 0.04 g/cm³. The carbon fiber was foundto have a specific resistance of 0.03 Ωcm when being compressed so as tohave a bulk density of 0.8 g/cm³.

The thus-produced carbon fiber was observed under an electronmicroscope, and the average diameter of fiber filaments of the carbonfiber was found to be about 150 nm. The branching degree of each fiberfilament was measured and found to be 0.13 occurrences/μm. The mass ofthe carbon fiber was measured, and carbonization yield (the mass of theproduced carbon fiber/the mass of the fed benzene) was calculated to be60%.

COMPARATIVE EXAMPLE 2

The vapor grown carbon fiber produced in Comparative Example 1 was firedat 1,000° C. for 15 minutes and then graphitized at 2,800° C. for 15minutes. The thus-graphitized carbon fiber was dispersed in polyacetalby use of a kneader to thereby prepare a compound. The vapor growncarbon fiber was added to the resin in an amount of 5 mass %. Theresultant compound was molded into a product by use of a thermal press,and the volume resistivity of the molded product was measured by meansof the four-terminal method and found to be 100 Ωm.

1. A vapor grown carbon fiber, each fiber filament of the carbon fiberhaving a branching degree of at least 0.15 occurrences/μm.
 2. A vaporgrown carbon fiber characterized by comprising carbon fiber filaments,each having a branching degree of at least 0.15 occurrences/μm, in anamount of at least 10 mass %.
 3. A vapor grown carbon fiber having abulk density of 0.025 g/cm³ or less.
 4. The vapor grown carbon fiberaccording to claim 1 or 2, which has a bulk density of 0.025 g/cm³ orless.
 5. The vapor grown carbon fiber according to any of claims 1 to 3,which, when compressed so as to have a bulk density of 0.8 g/cm³, has aspecific resistance of 0.025 Ωcm or less.
 6. The vapor grown carbonfiber according to any of claims 1 to 3, each fiber filament of thecarbon fiber having a diameter of 1 to 500 mm.
 7. The vapor grown carbonfiber according to any of claims 1 to 3, which is produced by feeding araw material solution containing a carbon source and a transitionmetallic compound into a reaction zone through spraying at a spray angleof 3° to 30° and subjecting the raw material solution to thermaldecomposition.
 8. The vapor grown carbon fiber according to any ofclaims 1 to 3, which is produced by feeding a raw material solutioncontaining a carbon source and a transition metallic compound into areaction zone through spraying, while feeding a carrier gas through atleast one site other than an inlet through which the raw materialsolution is sprayed, and subjecting the raw material solution to thermaldecomposition.
 9. A method for producing a vapor grown carbon fibercomprising spraying a raw material solution containing a carbon sourceand a transition metallic compound into a reaction zone and subjectingthe raw material solution to thermal decomposition, characterized inthat the raw material solution is sprayed at a spray angle of 3° to 30°.10. The method for producing a vapor grown carbon fiber according toclaim 9, wherein droplets of the raw material solution have an averagediameter of at least 5 μm.
 11. The method for producing a vapor growncarbon fiber according to claim 9 or 10, wherein the raw materialsolution and a carrier gas are fed through a concentric multi-tubenozzle into a reaction tube.
 12. The method for producing a vapor growncarbon fiber according to claim 11, wherein the raw material solution isfed through one of the tubes of the multi-tube nozzle, and another tubeserves as a passage for the carrier gas only.
 13. The method forproducing a vapor grown carbon fiber according to claim 12, wherein theraw material solution and the carrier gas are fed through the inner tubeof concentrically disposed two tubes, and the carrier gas is fed throughthe outer tube of the tubes.
 14. The method for producing a vapor growncarbon fiber according to claim 12, wherein the carrier gas is fedthrough the innermost tube and the outermost tube of concentricallydisposed three tubes, and the middle tube of the tubes serves as apassage for the raw material solution only.
 15. A method for producing avapor grown carbon fiber comprising spraying a raw material solutioncontaining a carbon source and a transition metallic compound into areaction zone and subjecting the raw material solution to thermaldecomposition, characterized in that a carrier gas is fed through atleast one site other than an inlet through which the raw materialsolution is sprayed.
 16. The method for producing a vapor grown carbonfiber according to claim 15, wherein the raw material solution issprayed at a spray angle of 3° to 30°.
 17. The method for producing avapor grown carbon fiber according to claim 9 or 15, wherein the rawmaterial solution containing a carbon source and a transition metalliccompound further contains a surfactant and/or thickening agent.
 18. Themethod for producing a vapor grown carbon fiber according to claim 9 or15, which comprises heating and firing recovered carbon fiber in anon-oxidative atmosphere at 800° C. to 1,500° C. and subsequentlyheating the thus-fired carbon fiber in a non-oxidative atmosphere at2,000 to 3,000° C., to thereby graphitize the carbon fiber.
 19. Themethod for producing a vapor grown carbon fiber according to claim 18,wherein, before being graphitized through heating, the recovered carbonfiber is doped with at least one boron compound, serving as acrystallization facilitating compound, selected from the groupconsisting of boron, boron oxide, boron carbide, a boric ester, boricacid or a salt thereof, and an organic boron compound in an amount of0.1 to 5 mass % as reduced to boron.
 20. A composite material comprisinga vapor grown carbon fiber according to any of claims 1 to
 3. 21. Acomposite material comprising a vapor grown carbon fiber producedthrough a method according to any of claims 9 or
 15. 22. A resincomposition comprising a vapor grown carbon fiber according to any ofclaims 1 to
 3. 23. A resin composition comprising a vapor grown carbonfiber produced through a method according to any of claims 9 or 15.